Switch to docker buildx for conformance image

The conformance test image previously used the `FROM --platform …`
syntax which is something we (SIG Release) consider as deprecated.

Therefore we now switch to `docker buildx`, which can specify the
`--platform` directly.

Signed-off-by: Sascha Grunert <mail@saschagrunert.de>

cluster/images/conformance/Dockerfile

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-FROM --platform=linux/ARCH BASEIMAGE
+FROM BASEIMAGE
 
 COPY ginkgo /usr/local/bin/
 COPY e2e.test /usr/local/bin/

cluster/images/conformance/Makefile

@@ -54,9 +54,14 @@ endif
 	chmod a+rx ${TEMP_DIR}/e2e.test
 	chmod a+rx ${TEMP_DIR}/gorunner
 
-	cd ${TEMP_DIR} && sed -i.back "s|ARCH|${ARCH}|g; s|BASEIMAGE|${BASEIMAGE}|g" Dockerfile
+	cd ${TEMP_DIR} && sed -i.back "s|BASEIMAGE|${BASEIMAGE}|g" Dockerfile
 
-	docker build --pull -t ${REGISTRY}/conformance-${ARCH}:${VERSION} ${TEMP_DIR}
+	docker buildx build \
+		--platform linux/${ARCH} \
+		--load \
+		--pull \
+		-t ${REGISTRY}/conformance-${ARCH}:${VERSION} \
+		${TEMP_DIR}
 	rm -rf "${TEMP_DIR}"
 
 push: build